1. Field of the Invention
The present invention relates to a semiconductor laser device driving circuit, and, more particularly, relates to a semiconductor laser device driving circuit of a light transmitter using a semiconductor laser device, used in a light communication system.
2. Discussion of the Related Art
FIG. 1 (PRIOR ART) is a block diagram illustrating an example of a conventional semiconductor laser device driving circuit. The semiconductor laser device driving circuit in FIG. 1 is arranged so that output light of a laser diode 1 is monitored by a photodiode 2 and a current made to flow into the laser diode 1 is changed to make the maximum and minimum values of the luminous output of the laser diode 1 always constant. The semiconductor laser device driving circuit having such a configuration intends to correct reduction of the luminous output of the laser diode 1 as the temperature increases during the operation thereof. That is, in the semiconductor laser device driving circuit in FIG. 1, an input signal DATA and an inverted input signal DATA* (the mark * represents an inverted signal) are applied to the respective bases of transistors Q.sub.1 and Q.sub.2 through a buffer amplifier 3. The laser diode 1 is inserted in the forward direction between a power supply V.sub.cc and the collector of the transistor Q.sub.1.
The collector of the transistor Q.sub.1 is connected to the collector of a transistor Q.sub.4 through a coil 14, and the collector of the transistor Q.sub.2 is connected to the power supply V.sub.cc. The respective emitters of the transistors Q.sub.1 and Q.sub.2 are connected to the collector of a transistor Q.sub.3 in common, and the emitter of the transistor Q.sub.3 is connected to the ground through a resistor R.sub.1.
An amplifier 7 amplifies the output of the photodiode 2 and a maximum value detection circuit 10 is supplied with the output of the amplifier 7. An amplifier 8 current-to-voltage converts a reference current I.sub.1. A differential amplifier 12 is supplied with the respective outputs of the maximum value detection circuit 10 and the amplifier 8, and the output of the differential amplifier 12 is applied to the base of the transistor Q.sub.3. On the other hand, a minimum value detection circuit 11 is supplied with the output of the amplifier 7. An amplifier 9 current-to-voltage converts a reference current I.sub.2. A differential amplifier 13 is supplied with the respective outputs of the minimum value detection circuit 11 and the amplifier 9, and the output of the differential amplifier 13 is applied to the base of the transistor Q.sub.4, the emitter of which is connected to the ground through a resistor R.sub.2. Further, one end of the photodiode 2 is connected to the power supply V.sub.cc.
In the above-mentioned semiconductor laser device driving circuit, the value of a modulation current made to flow into the laser diode 1 is determined by the output of the differential amplifier 12 supplied with the maximum value of the output current of the photodiode 2 and the reference current I.sub.1. In addition, the value of a bias current made to flow into the laser diode 1 is determined by the output of the differential amplifier 13 supplied with the minimum value of the output current of the photodiode 2 and the reference current I.sub.2. Therefore, as for the luminous output of the laser diode 1, the maximum and minimum light outputs thereof are determined by the reference currents I.sub.1 and I.sub.2 respectively, and the average light output and extinction ratio thereof are determined by the reference currents I.sub.1 and I.sub.2.
In the above-mentioned conventional semiconductor laser device driving circuit, the very small reference currents I.sub.1 and I.sub.2 which determine the luminous output of the laser diode 1 are current-to-voltage converted, and directly supplied to the differential amplifiers 12 and 13 respectively. Since the reference currents I.sub.1 and I.sub.2 are very small currents, they are apt to be affected by fluctuations in voltage of the power supply, external noises or the like so that their current values are changed easily. There is therefore a problem that the laser light output is likely to be changed because of fluctuations in voltage of the power supply, external noises or the like.